This is, in essence, a dependent clause, which is qualifying the independent clause in which it is contained. What this means is that these lines provide additional information about the greater context. We need to look at both.
The lines describe those who are captives marched on the metaphorical journey to Hell, much like Dante's Inferno. In fact, in this poem, Dante is the traveling observer. The captives area those who have grown old seeking power (120-121), those who have died young through their own actions or caused their own suffering (121-122), all those who continually tried to be famous (125-127) and basically everyone who would not follow God ("the Conqueror") (127-128).
Now let's look at the greater context: all of these people (the majority of all people) are basically destroyed by the moving Chariot of Life. This Chariot is driven by a shapeless being who seems to draw all the types of people mentioned in the specific lines above to follow him in a confused mass. You might compare this to the story of the Pied Piper. Overall. only those who truly follow God are free from the Shape and the Chariot. The poem asserts that there are very few of these.